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II
4/i III;1;
312         ON THE  CHANGE OF BEFBANGIBILITY  OF LIGHT.
69. I ought here to mention that a similar separation did not take place on the addition of water only to an ethereal extract from archil previously dried. The condition which determined the separation in the first case appeared to be the presence of a small quantity of ammonia, which would evaporate on drying the archil. And in fact when a small quantity of ammonia was added to the extract from dried archil, a partial separation was effected. I do not here enter into the question whether one of the sensitive principles may be obtained from the other, whether, for example, a chemical combination of the orange-dispersing principle with ammonia might disperse a green, or a green with a little orange. A solution containing a mixture of the same substance in two different states of chemical combination, both compounds being sensitive, is not the less justly regarded as containing two distinct sensitive principles.
!                                70.   The preceding results are mentioned, not for their own
!                           sake, but merely for the sake of the method of examination em-
I                           ployed.    The results indeed are so imperfect as to be worthless on
|                           their own account.    A complete optico-chemical examination of
archil and litmus would itself alone furnish a subject for research of no small extent;  but it belongs rather to chemistry than to general physics.    It is quite possible that internal dispersion may i.11                            turn out of importance as a chemical test.    The dispersing such a
il                            tint, and the having the dispersed light produced by light of such
f                            a refrangibility, form together a double character of so peculiar a
!5'                                nature that it enables us, so to speak, to see a sensitive principle
fy                              in a solution containing many substances, some of them, perhaps,
>\                            coloured, so that the colour of the solution may be very different
']!                             from what it would be if the sensitive principle were present alone.
l!
j                                  71.    The law mentioned at the beginning of Art. 63 did not
|                            seem very applicable to archil when the fluid was merely diluted
1                             with water.   But when the orange-dispersing and green-dispersing
!j                            principles were   obtained, as it would   appear, more nearly in a
state of isolation, by means of ether and water, the law was found
to be obeyed.    Thus, when the ethereal solution which exhibited
.'I                            the orange dispersion and little else was examined by the third
method, the dispersion was found to commence with a tail of light
(j                            followed by a dark tooth, indicating   the position of a band of